1. Field of the Invention
This invention relates to a transmission mounted on a vehicle, such as an automobile. The vehicle transmission generally includes an automatic transmission, a manual transmission or the like, such as a transaxle mounted on a front engine front wheel drive (FF) vehicle or a transmission mounted on a front engine rear wheel drive (FR) vehicle.
2. Description of the Related Art
When a vehicle transmission is operating, an oil pump sucks oil existing in an inner bottom portion of a casing and supplies the sucked oil to appropriate portions of the transmission.
In some cases, when a vehicle transmission is operating, oil is splashed up from an inner bottom portion of a casing by an appropriate rotating gear disposed inside the casing, and is guided to the suction side of an oil pump, a portion that particularly requires lubrication, or the like.
In this case, because the oil in the casing is delivered or conveyed to the downstream side by the rotation of the gear, the oil is not likely to be equally distributed in the internal space of the casing. In particular, as the gear rotates more rapidly, oil is delivered or conveyed more rapidly to the downstream side by the rotation of the gear, thereby enhancing the inequality in the oil distribution and increasing the temperature of the oil and the inner pressure of the casing.
Usually, to reduce the above-described increase in the inner pressure of the casing, an air release hole that releases air to outside is provided on the upper side of the casing.
To restrict or prevent the oil from being discharged from the air release hole, a breather plug is attached to the air release hole, or a breather chamber having a maze-like structure is provided to separate oil from air. (See, for example, Japanese Patent Application Publication Nos. 2005-291263 and 10-205609)
In the meantime, in the case where a casing of a vehicle transmission has a partitioned structure formed by combining multiple parts, the internal space of the casing may be divided into multiple sections, though the sections are mutually communicated.
In this case, when the vehicle transmission is operating, along with the oil delivering operation by the gear, a phenomenon may occur in which oil is likely to accumulate in a space positioned on the downstream side of the rotation direction of the gear (i.e., on the downstream side of the oil flow generated by the rotation of the gear), and air is likely to accumulate in a space positioned on the upstream side of the rotation direction of the gear (i.e., on the upstream side of the oil flow generated by the rotation of the gear).
In consideration of the phenomenon, the air release hole is typically provided in the ceiling wall of the casing on the side of the space where the oil is likely to accumulate.
In the above-described technologies, when the gear rotates at a high speed, the oil delivery by means of the gear becomes faster. Accordingly, in the space in which the oil is likely to accumulate (i.e., the space on the downstream side of the rotation direction of the gear), the oil is raised or increased and forms bubbles. On the other hand, in the space in which air is likely to accumulate (i.e., the space on the upstream side of the rotation direction of the gear), the inner pressure is likely to increase.
In this case, because the air release hole is provided above the space in which oil is likely to accumulate, it is difficult to release the inner pressure of the space in which air is likely to accumulate through the air release hole.
Due to the above, the inner pressure of the space in which air is likely to accumulate further increases, and the oil or its bubbles in the space in which oil is likely to accumulate is/are urged to be further raised. Therefore, in an extreme case, oil may leak to outside from the breather plug, or oil may passes through the breather chamber and leaks to outside from the air release hole.